Apparatus for carbon content analysis

ABSTRACT

An enclosure of metal, in which carbon is both soluble and mobile, is inserted in the fluid in which carbon content is to be determined. Within the enclosure is an electrolyte and a reference electrode dipping into the electrolyte. The potential between the reference electrode and the metal of the enclosure provides an indication of the carbon content in the fluid.

United States Patent [1 1 Long [ APPARATUS FOR CARBON CONTENT ANALYSIS Inventor: Geoifrey Long, Abingdon, England United Kingdom Atomic Energy Authority, London, England Filed: June 9, 1970 Appl. No.: 44,702

Assignee:

[30] Foreign Application Priority Data June 13, 1969 Great Britain ..l 0... 30,2 l0/69 US. Cl 204/195 R, 204/1 T 1111.01. 60111 22/46 Field of Search 204/] T, 195

[56] References Cited UNITED STATES PATENTS 3,403,090 9/1968 Tajiri et ai. 204/195 8/1970 Morris et al. 204 195 Primary Examiner-T. Tung Attorney-Larson, Taylor and Hinds [5 7] ABSTRACT An enclosure of metal, in which carbon is both soluble and mobile, is inserted in the fluid in which carbon content is to be determined. Within the enclosure is an electrolyte and a reference electrode dipping into the electrolyte. The potential between the reference electrode and the metal of the enclosure provides an indication of the carbon content in the fluid.

4 Claims, 1 Drawing Figure 1 APPARATUS FOR CARBON CONTENT ANALYSIS BACKGROUND'QF THE lNVENTlON The present invention relates to a device for measuring the carbon content of fluids. 1

It is .known that when, for example, steel articles have been made, the process of manufacture frequently introduces work hardening which must be relieved, and to this end the articles are normally annealed for an appropriate length of time. The atmosphere in the annealing furnace'must be such as to cause no carburising or decarburising of the steel and is normally provided by the incomplete combustion offuel gas. The atmosphere therefore contains, in practice, methane, carbon monoxide, carbon dioxide, hydrogemand water vapour, the various constituents being present in a complex equilibrium state. In order to ensure that the carburising potential of the atmosphere is correct, it is necessary to measure this potential from time to time but unfortunately this has hitherto proved difficult, since existing methods, for example chemical analysis of the gas atmosphere, give data which are only indirectly related to the required carburising potential. It is therefore extremely desirable to provide a device for measuring the effective carbon content of the atmosphere within the furnace. I

It is'also known that the presence of carbon in liquid metals may cause damage or create difficulties. For example, the presence of carbon in liquid sodium (which is used as a coolantin nuclear reactors) may cause carburising ofsteel structural parts of the reactor. To

avoid this it is necessary continuously to monitor the carbon content of the molten sodium so that when necessary a proportion may be withdrawn and passed to a clean-up system. There is therefore a need'for a monitor for the carbon content of sodium in nuclear reactors. The above example is given purely by way'of explanation. There are other environments ,where it is desired to measure the carbon content of a fluid, for example in the steel industry it would be very convenient to perform a rapid analysis for carbon upon the molten steel. i

In U.K. Pat. No. 1,132,753 (U.S. Pat. No. 3,523,066) there is described a method of measuring the carbon content of a metal comprising setting up the cell: metal containing-carbon/salt-electrolyte/graphite; and measuring the electromotive force at a known temperature below the melting point of the metal, the electrolyte containing carbon in an ionised state and having a known carbon potential relative to graphite. There is also described apparatus for measuring the carbon con- SUMMARY OF THE INVENTION The present invention provides a device for measuring the carbon content of fluids, which in'part makes use of the techniques described in the said U.S. Pat. No. 1,132,753.

According to the present invention there is provided a device for measuring the carbon content of fluids comprising an enclosure for insertion into the fluid, the enclosure comprising a metal in which carbon is both soluble and mobile at the operating temperature; a reference electrode within the enclosure, the said reference electrode containing a known proportion of carbon; means for making electrical connection to the enclosure which forms a second electrode; a salt electrolyte within the enclosure and in contact with the inner surface thereof and the said reference electrode, the electrolyte containing carbon in an ionised state and having ,a known carbon potential relative to thesaid reference electrodejand means for measuring the potential difference between the enclosure and the said reference electrode. i

i The most convenient reference electrode is carbon dispersed in a metal matrix with a known proportion of carbon content, or the electrode may comprise graph ite provided precautions are taken to prevent free carbon therefrom from migrating to the enclosure.

It will be appreciated that the aforesaid device involves the setting up of the electrochemical cell:

metal containing carbon/electrolyte containing carbon/graphite.

The output of this cell is given by the formula:

wherein:- E is the electromotive force,

R is the gas constant,

T is the absolute temperature, F is the Faraday equivalent,

2 is the valency of the carbon ions in the electrolyte,

and I a,- is the activity of the carbon in the metal relative to graphite.

It will be apparent that the output of the cell depends on temperature and therefore it may be desirable to operate with a molten electrolyte but this is not strictly necessary. However, the use of a molten electrolyte does ensure good electrical contact between the electrolyte, the electrode and the enclosure.

Calcium carbide is a suitable electrolyte, but since it melts at 2,300C, a solvent is desirable. For a steel thimble, calcium chloride is a suitable solvent and the use of a 10 mol per cent solution'of calcium carbide in calciurnchloride is satisfactory. Any other stable mol ten salt, in which calcium carbide is soluble, would be suitable in place of the calcium chloride.

It is convenient to use very pure calcium chloride which is melted and dried, at a temperature higher than that at which it will be used, in a pure inert gas atmosphere. The calcium carbide is cnveniently prepared by reacting pure calcium with graphite in an inert gas atmosphere.

If the present invention is to be used for measuring the carbon content of molten sodium or a furnace atmosphere, the enclosure may comprise iron, nickel or their alloys which are all materials permeable by any hydrogen present in the gas atmosphere. Thus hydrogen will penetrate to the interior of the enclosure, but this is no disadvantage in the device according to the present invention.

The equilibrium reactions taking place within a furnace can be represented as:

2 CO 7 CO2 C CH4 2H C I H O H l/2O The equilibrium positions for these reactions depend on the redox and carburising potential within the furnace. Of the species present in the furnace atmosphere hydrogen and carbon will pass through the enclosure, the rate of passage of the hydrogen depending on the material of the enclosure but being very fast in the case of an iron enclosure. v

In the case of molten sodium, there will not normally be other species than carbon present and which might migrate through the enclosure.

In the case of molten steel, the problem of selecting a material for the enclosure is acute by virtue of the intense solvating action of steel and slag, but it is though that such a metal can be found which will be resistant for a sufficient length of time to permit measurements to be made.

The measurement is an equilibrium measurement. Once equilibrium is reached the carbon content concentration in the metal of the enclosure will depend in a calculable manner on the carbon concentration of the fluid in contact with the outer surface of the enclosure. Whilst the enclosure is maintained in contact with the fluid, changes in the carbon concentration of the fluid will cause a corresponding change in concentration in the metal of the enclosure and, consequently, in the potential between the enclosure and the said first electrode. Thus, continuous monitoring of carbon content of the fluid may be provided.

DESCRIPTION OF PREFERRED EMBODIMENT A specific construction of device embodying the in-' vention will now be described by way of example with reference to the accompanying drawing which is a diagrammatic sectional view of the device.

In the drawing is represented diagrammatically a furnace 11 into which a fluid 12 of unknown carbon content flows via inlet 13. The fluid 12 leaves the furnace 11 by outlet 14.

An enclosure is provided by a thin-walled thimble 15 of metal appropriate (as explained above) to the nature of the fluid 12. The thimble 15 is sealingly secured to a thickerwalled support tube 16, mounted so that the thimble 15 is within the furnace and projecting into the fluid 12, whilst the support tube 16 extends out, with an appropriate seal (not shown), through an aperture in the furnace 11.

The top of the support tube 16 is closed by an electrically insulating closure 17 which supports a reference electrode 18.

The reference electrode 18 desirably comprises carbon dispersed in a metal matrix and extends centrally through the closure 17 down to just short of the bottom of the thimble 15. A pure graphite reference electrode may be employed, but an electrode comprising carbon dispersed in a metal matrix has the advantage that the carbon is firmly bound and the risk of carbon particles ff'om the reference electrode migrating to the thimble surface is avoided.

In the thimble 15 is the electrolyte 19 which is in contact with the interior surface of the thimble l5 and with the reference electrode 18.

A potentiometer 21 is provided for measuring the potential difference between the thimble 15 and the reference electrode 18.

in operation, carbon from the fluid 12 migrates into the metal of the thimble 15 and, at equilibrium, the concentration of carbon in the walls of the thimble is related in a calculable manner with the concentration of carbon in the fluid surrounding the thimble 15. The reading of the potentiometer 21 is related to the carbon concentration in the metal of the thimble 15 and thus provides an indication of carbon content in the fluid.

Clearly there is a time lag between a change in carbon content of the fluid and the corresponding indication on the potentiometer, but in practice the time lag is short enough for statisfactory continuous monitoring of a flowing fluid.

The invention is not restricted to the details of the foregoing example.

1 claim:

1. A device for measuring the carbon content of fluids comprising an enclosure for insertion into the fluid, the enclosure comprising a metal in which carbon is both soluble and mobile such that the concentration of carbon in the enclosure metal will depend on the carbon content of the fluid into which it is inserted; a reference electrode within the enclosure, the reference elctrode comprising carbon dispersed in a metal matrix and containing a known proportion of carbon; means for making, electrical connection to the enclosure which forms a second electrode; I a salt electrolyte within the enclosure and in contact with the inner surface thereof and the said reference electrode,-the electrolyte containingcarbon in an ionized state and having a known carbon potential relative to the said reference electrode; and means for measuring the potential difference between the enclosure and the said reference elctrode.

2. A device as claimed in claim 1, wherein the electrolyte comprises calcium carbide dissolved in a salt which is molten at the operating temperature.

3. A device as claimed in claim 2, wherein the said salt comprises calcium chloride.

4. A device as claimed in claim 1, wherein the enclosure is of a material selected from iron, nickel, an alloy of iron, and an alloy of nickel. 

2. A device as claimed in claim 1, wherein the electrolyte comprises calcium carbide dissolved in a salt which is molten at the operating temperature.
 3. A device as claimed in claim 2, wherein the said salt comprises calcium chloride.
 4. A device as claimed in claim 1, wherein the enclosure is of a material selected from iron, nickel, an alloy of iron, and an alloy of nickel. 